NHBC have published an update to their 2007 technical guidance for low-rise residential developments on sites affected by ground gases. The original document, entitled ‘Guidance on evaluation of development proposals on sites where methane and carbon dioxide are present’, included a simple multi stage classification method for low-rise housing, commonly referred to as the “Traffic Light system”.  The fundamental guidance offered in this document remains applicable but, since its publication, there have been a number of advances in knowledge, including guidance on alternative approaches for characterising gas regimes and updated advice on the design of measures to deal with gas risks.  These include the Characterisation of gas risks without gas monitoring on low-risk sites (CL:AIRE, 2012), Verification of gas protection systems (CIRIA C735, 2014) and the British Standard Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings BS8485:2015.

The full article is available at: http://www.nhbc.co.uk/Builders/ProductsandServices/TechZone/NHBCStandards/TechnicalExtra/

NHBC summarise the new guidance and comment on where they consider it to be applicable.  They also explain where their Traffic Light classification can continue to be used, for a “typical house”, as detailed in the figure below.

Model residential property developed for calculated maximum permitted gas concentration within the subfloor void.

They advise that:

• There have been a number of recent UK publications offering alternative approaches to ground gas risk assessment and improved advice for the design and verification of measures to deal with gas risks.
• Practitioners undertaking gas surveys and assessing the risks should be conversant with updated guidance.
• Robust site characterisation is required to design gas protection measures, but designers must also have an understanding of building-related influences, as these significantly govern design and construction options for gas protection measures.
• Gas protection design, installation approach and verification requirements should be agreed with NHBC in advance of works, as satisfying requirements after construction is extremely difficult, often more costly, and can be disruptive.
• Specific requirements relating to gas protection measures may be applied under planning and must also be considered.
• The NHBC Traffic Light guidance can be used where the development proposals are based on the ‘typical house’ used for modelling in the traffic light classification system.
• Verification evidence will be requested for gas regimes at Amber 2. For developments where the Characteristic Situation is applicable, the BS8485 scoring system requirements should be adopted, and verification evidence could be required for gas regimes at CS2 or above.

References

NHBC Technical Extra, Issue 20, April 2016.

Contaminated Land: Applications in Real Environments (CL:AIRE) Research Bulletin RB17 – A pragmatic approach to ground gas risk assessment. 2012.

CIRIA C735. Good Practice and verification of protection systems for buildings against hazardous ground gases. 2014.

BSI. Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings, BS8485:2015.

by Neil Parry